Liquid liquid phase separation and tau biology Tau is a component of neurofibrillary tangles in Alzheimer disease and of analogous aggregates in closely related neurodegenerative diseases called tauopathies. What initiates a change from a very soluble, microtubule associated protein to an aggregate is unknown. The current application is based on a series of very recent experiments that may put a new twist on decades of observations of tau. In the first aim we further explore the new observation that tau (if phosphorylated, mutant, or exposed to polyanions like RNA) can undergo liquid liquid phase separation forming a coherent highly concentrated and reversible droplet. Intriguingly, liquid-liquid phase separation has currently been implicated in the pathobiology of Fus, TDP43, hnRNP, and C9orf72 proteins in ALS; with our new data on tau and Alzheimer's and frontotemporal dementia, the analogy that liquid liquid phase transitions underlie some aspects of neurodegeneration is compelling to explore. The structure of these droplets is fascinating, and over time they undergo a gelation and finally remodel to have some beta pleated sheet structure, potentially on the pathway towards the highly ordered structure of neurofibrillary tangles. The second aim examines the function of these droplets which, when ?fresh?, have the ability to interact with tubulin and to nucleate microtubule structures, ?wetting? the surface of the microtubule bundle and then flowing along it, stabilizing the extending structure. Given the long history of studies showing conclusively that tau stabilizes microtubules, it seems inescapable that droplet formation is linked to this fundamental physiological role of tau. The third aim is to bring these observations full circle, from in vitro observations with recombinant proteins to the tau that is present in the brain ? both in animal models of tau induced neurodegeneration and in human Alzheimer, Frontotemporal dementia (tau positive or TDP43 positive), and control brain. Our preliminary data show that soluble tau isolated from Alzheimer brain ? using biochemical separation techniques to enrich for high molecular weight hyperphosphorylated oligomers and an immunoaffinity column, retains the ability to form LLPS. Our proposed studies will further examine this fraction of tau, compare its properties to other fractions, and (together with aims 1 and 2) allow us to explore how reduced systems studying tau relate to the tau present in human disease. We suggest that the newly described liquid liquid phase separation properties of tau are an important clue that connects the structure and function of tau ? and phosphorylated and mutant forms - with the disease relevant initiation of aggregation.